1. Field of the Invention
The present invention relates to a memory device having a nonvolatile memory, such as a write once memory device.
2. Description of the Related Art
In modern society, where many electronic devices are used, various data are generated and used; therefore, memory devices are required to store the data. Various memory devices produced and used today have different advantages and disadvantages, and are used properly depending on the data to be stored and used.
For example, a volatile memory that loses its memory content when the power is turned off includes a DRAM and an SRAM. The applications of volatile memories are significantly limited because of the volatility; however, each of them is used as a main memory device or a cash memory of a computer taking advantage of a short access time. Since a DRAM has small memory cells, a large-capacity DRAM can be produced easily. However, it is controlled in a complex manner and consumes much power. An SRAM includes a memory cell formed from a CMOS transistor and is easily manufactured and controlled; however, it is difficult to obtain a large-capacity SRAM since one memory cell needs six transistors.
A nonvolatile memory that holds its memory content even after the power is turned off includes: a rewritable memory into which data can be rewritten many times; a write-once memory into which data can be written by a user only once; and a mask ROM of which data content determined in the manufacturing of the memory cannot be rewritten. As the rewritable memory, there are an EPROM, a flash memory, a ferroelectric memory, and the like. The EPROM allows an easy writing of data and unit cost per bit is relatively low; however, a program device and an eraser dedicated to writing and erasing are required. The flash memory and the ferroelectric memory can be rewritten on a substrate used, have a short access time, and consume less power; however, steps for manufacturing a floating gate and a ferroelectric layer are required. Thus, the unit cost per bit is high.
Each memory cell of a write-once memory includes a fuse, an antifuse, a cross pointer diode, an OLED (Organic Light Emitting Diode), a bistable liquid crystal element, and other devices whose states are changed by heat or light. Further, in recent years, memory elements using organic materials are actively developed (for example, Reference 1: Japanese Patent Application Publication No. 8-116109).
Further, in the case of using a structure in which an insulator is sandwiched between conductive films serving as a cathode and an anode, which is provided over a thin film transistor for a memory cell of a write once memory, the layer of the insulator sandwiched between the cathode and the anode is formed by a droplet discharge method, vapor deposition, or the like separately from an interlayer insulating film formed over the thin film transistor, which creates problems of increase in cost and the number of manufacturing steps.